What you need to do is decode it to 6 wave channels, slow it down (of course), and then re-encode back to DTS.

Mind you, there are those that say 640kbps DD is pretty much sonically equivalent to 768kbps DTS, so if the source isn't full 1536kbps, it's a good alternative.

For DTS to AC3, here is a nice guide: http://forum.doom9.org/showthread.php?t=91239

If you really want to keep with DTS, here's all the info you need on working with it: http://forum.doom9.org/showthread.php?s=&threadid=68300#post424069